1. Field of the Invention
The present invention relates to a radio apparatus such as a portable radiotelephone apparatus.
2. Description of the Related Art
In recent years, an increasing number of portable radiotelephone apparatuses (hereinafter referred to as portable telephone apparatuses) have been used. In addition, portable telephone apparatuses that are small and that have high performance have been aggressively developed.
Current developing trends of portable telephone apparatus are for example small size for high portability, low power consumption for long time operation, and high linearity for high resistance against disturbing waves.
Currently, studies for solving problems necessary to accomplish such features have been performed.
Next, problems of the current portable telephone apparatus will be described in the order of a receiving portion, a synthesizer (namely, a local oscillator), a transmitting portion, and an antenna.
First, problems of the receiving portion will be described.
The receiving portion has two problems. As a first problem, the current consumption of the portable telephone apparatus is large. As a second problem, when a signal is received, a DC offset takes place, resulting in causing the reception characteristic of the portable telephone apparatus to deteriorate.
Since the reception characteristic of the portable telephone apparatus should always satisfy the required performance, the reception characteristic is designated so that the portable telephone apparatus properly operates in the worst radio wave environment. An example of the worst radio wave environment is a situation of which an unnecessary signal defined as a mutual modulation characteristic or a selectivity of adjacent channels is present. In other words, when an unnecessary signal other than a necessary signal is present in a system band, the level of the unnecessary signal is the maximum value of which a desired bit error rate defined in the system is satisfied.
Generally, to satisfy the standard value of the system in the worst radio wave environment, the radio apparatus should properly operate in the worst condition. Thus, in other than the worst radio wave environment, the portable telephone apparatus operate with the performance that satisfies the worst condition. To satisfy the standard in case of the worst radio wave environment, the receiving portion of the portable telephone apparatus should have linearity. In other words, the distortion of the receiving portion should be decreased so that the standard is satisfied. This problem relates to currents that flow in circuit blocks of the receiving portion (such as a low noise amplifier and a frequency converter).
Generally, to improve the linearity of a circuit, the operating current thereof should be increased. Thus, the power consumption of the portable telephone apparatus considered for the worst radio wave environment excessively increases. This is because the portable telephone apparatus is not always in the worst radio wave environment. In other words, the portable telephone apparatus normally operates in other than the worst radio wave environment.
Next, the second problem of the receiving portion (namely, when a signal is received, a DC offset causes the reception characteristic to deteriorate.
Generally, in an active circuit such as a frequency converter, a low frequency filter, or a low frequency amplifier used in the receiving portion of the portable telephone apparatus, the output signal thereof overlaps with a desired signal, thereby generating a DC component. Such a DC component is generated by a self-mixing operation.
As the simplest technique for removing the DC component, an AC coupling capacitor may be connected to the output stage of the active circuit. In this case, part of the desired signal component is deleted. In other words, a notch takes place.
Thus, a carrier-to-noise (C/N) characteristic may be improved for an FSK signal with a high modulation index of which a desired signal component is small in the vicinity of the DC region.
A technique for removing a DC offset using an AC coupling capacitor has been proposed. This technique can be effectively used for a two-value FSK signal with a high modulation index for pagers. Since a signal component in the vicinity of the DC region is small, the AC coupling capacitor does not largely attenuate a signal component.
However, in an FSK signal and a four-value FSK signal that have been used for high speed data transmission in recent years and that have low modulation indexes, since there are many signal components in the vicinity of the DC region, the second problem cannot be practically solved.
Such a DC offset that takes place in the receiving portion has a problem in the heterodyne system. This problem is much serious in the direct conversion system that has been used in the mobile communication field in recent years. The problem of the DC offset in the direct conversion system has different features from the problem in the heterodyne system. Next, the features of the problem in the direct conversion system will be described.
In the direct conversion system, an external radio signal (RF signal) and a local signal with the same frequency thereof are sent to a mixer so as to directly convert an RF signal into a baseband signal.
When the mixer is mathematically ideal, the isolation between each terminal is infinite. Thus, a signal supplied to a particular terminal does not take place at other terminals.
However, since a mixer used in the direct conversion type portable telephone apparatus does not have an infinite isolation, a local signal of the portable telephone apparatus is radiated from the antenna. The local signal radiated from the antenna is reflected by an external reflector. The reflected signal is received by the antenna and then sent to the mixer. Since the frequency of the signal that is sent to the mixer from the antenna is the same as the frequency of the local signal, a multiplying operation as a mixing function causes a DOC component (namely, a DC offset) to take place at a baseband output terminal.
Since the DC offset varies depending on the amount of reflection of the local signal (namely, a reflector in the vicinity of the antenna), this DC offset more adversely affects the reception characteristic than a DC offset of the portable telephone apparatus and a DC offset of an active device.
Since the direct conversion type portable telephone apparatus is small, the user carries it with his/her hand, bag, and pocket, the situation of an external reflector varies time by time. Thus, since the amount of reflection of a local signal varies time by time, the DC offset varies time by time. Since the DC offset cannot be suppressed, the reception sensitivity deteriorates.
To compensate the DC offset, a capacitor may be disposed in a downstream circuit. Since the capacitance of the capacitor is constant, a time-varying transient response of the DC offset largely affects a reception error rate.
Thus, the conventional receiving portion cannot solve the two problems with respect to the low current consumption and the improvement of the reception characteristic. In particular, the problems in the direct conversion system are severer than the problems in the heterodyne system.
Next, the problem of the synthesizer of the conventional portable telephone apparatus will be described.
In the conventional portable telephone apparatus, a frequency synthesizer is used. The frequency synthesizer comprises a reference oscillator, a reference frequency divider, a phase comparator, a loop filter, a VCO, and a comparing frequency divider. The frequency of the comparing frequency divider is varied from N1 to N2 so as to switch a frequency. The frequency switching time depends on a natural angular frequency ωn and a dumping coefficient ζ of the loop of the loop filter. When the natural angular frequency and dumping coefficient are selected for a stable oscillation frequency and low noise, the frequency switching time becomes long.
The frequency synthesizer of this type should have a low phase-to-noise characteristic, the frequency switching time becomes long. Thus, when the conventional frequency synthesizer is used for a TDMA type portable telephone apparatus, the apparatus cannot search a blank channel using a blank slot in the communicating state.
Next, the transmitting portion of the conventional portable telephone apparatus will be described.
The transmitting portion of the conventional portable telephone apparatus comprises a frequency converter, a variable attenuator, a power amplifier, a transmission power controlling circuit, a transmission/reception switch, a band pass filter, a directional coupler, and a power detector. The frequency converter, the variable attenuator, the power amplifier, the transmission power controlling circuit, the transmission/reception switch, and so forth can be easily structured as an IC device. Thus, the sizes of these structural parts have been decreased corresponding to the advancement of the IC technologies.
However, since it is difficult to structure the band pass filter and the directional coupler as an IC device, these parts should be mounted on a mother board.
For example, the directional coupler is a chip part with a size of 5 mm×5 mm. On the other hand, the power detector is structured as a diode switch having a diode, a capacitor, a resistor, and so forth. Due to the mounting areas of the diode, capacitor, resistor, and so forth, the size of the power detector exceeds 5 mm×5 mm.
Thus, unlike with the requirement of the size reduction, the volume of the portable telephone apparatus adversely increases. In addition, since the directional coupler wastes an output power, the output power of the power amplifier should be increased so as to compensate the wasted power. Consequently, the power consumption of the transmitting portion increases.
Next, the problems of the antenna of the conventional portable telephone apparatus will be described.
To improve the portability of the portable telephone apparatus, the sizes of the battery and antenna have been remarkably decreased. However, the size of the circuit of the portable telephone apparatus has not been sufficiently decreased. Thus, considering the decrease of the overall size of the portable telephone apparatus, the size of the antenna should be further decreased.
On the other hand, there are problems of the body of the user against the antenna. The body of the user absorbs or scatters a radio frequency wave. In addition, the body causes the operating impedance of the antenna to vary. From a view point of a radio frequency, the body functions as a radio wave absorber with a high dielectric constant. Thus, the body of the user causes the radiation characteristic of the antenna to deteriorate.
Since the size and thickness of the portable telephone apparatus have been decreased, the ear of the user tend to further approach to the antenna, resulting in causing the antenna characteristic to further deteriorate.
As one of factors of such a deterioration, the body of the user causes the impedance of the antenna to fluctuate. This situation will be described assuming that the antenna is used for transmitting a signal.
To cause the antenna to radiate a radio wave, a power should be supplied to the antenna. The optimum condition of the power supplied to the antenna is in that the impedance of the feeder line is equal to the impedance of the antenna. When the impedance of the antenna fluctuates from its optimum value, a power on the feeder line is reflected at the input edge of the antenna to the transmitting amplifier. This reflection sometimes causes the amplifier to oscillate.
Next, a technique that can solve such problems and that can be easily analogized and problems involved in the technique will be described.
To suppress the power from being reflected at the input edge of the antenna, the frequency band of the antenna is widened. In other words, even if the input impedance fluctuates due to the approaching of the body of the user, the fluctuation of the wide frequency band antenna is smaller than that of a narrow frequency band antenna. However, when the frequency band of the antenna is widened, the volume of the antenna should be increased. Thus, the technique for widening the frequency band of the antenna contradicts with the decrease of the size of the portable telephone apparatus.
As another technique for suppressing the reflection of the power at the input edge of the antenna, the impedance of the antenna is adjusted in such a manner that when the body of the user approaches the portable telephone apparatus the impedance becomes optimum. However, it cannot be said that this technique is not unconditionally good. This is because the portable telephone apparatus is not always used in the state that the body of the user approaches the portable telephone apparatus. Since the user carries the portable telephone apparatus with his/her hand or bag, the operation state thereof varies time by time. Thus, the amount of fluctuation of the impedance of the antenna varies corresponding to the operation state of the portable telephone apparatus. This is because the substance and distance of the body of the user to the portable telephone apparatus vary corresponding to the operation state thereof. When the amount of fluctuation varies, it is very difficult to optimally adjust the impedance of the antenna.
A part of the body that most approaches the antenna is an ear of the user. However, the size of the ears varies person by person. The difference of the size of the ears largely affects the performance of the antenna. The ears of the user cause the impedance of the antenna to largely fluctuate. This is because the dielectric constant of ears is as high as 80. When an ear of the user approaches the antenna, the electrical length of the antenna largely varies. Depending on whether or not an ear contacts the antenna or whether the ear is close to or far apart from the antenna, the impedance largely varies. The relative position of an ear to the antenna largely depends on the size of the ear. Thus, even if the impedance is optimized in the state that the body of the user approaches the antenna, the optimized antenna may be not optimum for other people. Thus, the performance of the antenna deviates person by person.
Besides the above-described techniques, there are several techniques for optimally controlling the matching circuit corresponding to the operation state.
As the first technique, a matching circuit of the antenna is switched to the other corresponding to the on/off state of a call button. This technique is based on the assumption that when the call button is turned on, an ear of the user is close to the antenna.
Although this technique can be accomplished with a simple structure, it cannot deal with the variation of the size of ears of each user.
As the second technique, the level of a wave reflected from the antenna is detected and a matching circuit of the antenna is switched to the other corresponding to the amount of reflection.
However, in this case, to detect the amount of reflection, it is necessary to place a probe between the antenna and the radio circuit. This probe may cause a reflection loss, a conductor loss, and/or a loss of an RF signal to take place.
Thus, according to the above-described conventional portable telephone apparatus, in the receiving portion, the maximum current should be always supplied. Thus, the current consumption is excessive large. When a DC offset is removed with an AC coupling capacitor, a desired signal component is also attenuated. In addition, there is a time-varying DC offset that is caused by a reflection of an external reflector and that cannot be removed by an AC coupling capacitor. Thus, the deterioration of the reception sensitivity cannot be suppressed.
In addition, the synthesizer cannot search a blank channel with a blank channel slot in the communicating state.
In the transmitting portion, the mounting sizes of circuit parts such as a directional coupler and a power detector other than an antenna are large. Thus, the size of the portable telephone apparatus cannot be further decreased.
In the antenna, when the user who carries the portable telephone apparatus approaches the antenna, the antenna characteristic deteriorates. To solve this problem, the size of the antenna should be increased. Alternatively, the user should be selected for the antenna.